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This study provides insights into the alternative use of barochromic studies for determining the polarizability of organic molecules in excited states.
A recent study published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy revealed that the solute–solvent repulsion effects on the absorption spectra of anthracene in n-hexane under high pressure significantly influence the band positions in the UV-vis absorption spectra (1). According to the researchers, the repulsive interactions were influenced by solvent polarity and Onsager cavity radius changes (1).
Anthracene is a polycyclic aromatic hydrocarbon widely used as a model compound in spectroscopic studies (1). It is known for its optical properties (1). Normally, dispersive solute–solvent interactions contribute to the absorption spectra of anthracene. This study demonstrates, however, that the repulsive interactions between the solvent and solute is also important, as it determines the band positions in the absorption spectra (1).
The researchers employed high-pressure techniques to investigate the effects of solute-solvent repulsion on the absorption behavior of anthracene in n-hexane. By subjecting the compressed solution to varying pressures, they observed significant changes in the absorption spectra. The strength of the repulsive interactions was found to be influenced not only by the solvent polarity, but also by the changes in the Onsager cavity radius because of pressure variations (1).
The results of the study show that repulsive interactions alongside dispersive interactions should be considered when interpreting barochromic and solvatochromic results of aromatic compounds (1). Barochromic refers to the phenomenon where the absorption wavelength of a compound shifts as a result of pressure changes (1). The complex interplay between solutes and solvents impacts the absorption phenomena that scientists do not yet fully understand. By incorporating repulsive interactions into the analysis, it is possible for scientists to gain a better understanding of how the solute and solvent interactions affect absorption (1). On the other hand, solvatochromic refers to the phenomenon where the absorption or emission properties of a molecule change in response to changes in the polarity or composition of the solvent (1). It is a term commonly used in spectroscopy to describe the shifts in the spectral characteristics of a compound because of variations in the surrounding solvent environment.
Additionally, the research suggests that barochromic studies can serve as a promising alternative to solvatochromic studies for investigating the polarizability of organic molecules in their excited electronic states. By subjecting the compressed solute-solvent system to different pressures, researchers can obtain valuable information about the changes in the absorption properties of the compound (1).
Another important aspect to this study is the pressure-induced polarity change in n-hexane. The study shows that the exchange of n-alkane solvents is surpassed by the polarity change in n-hexane (1). This finding shows that n-hexane operates differently under high pressure conditions, emphasizing the importance of considering solvent response to external factors (1)
Overall, this study is important in the field molecular spectroscopy because it expands our knowledge of the complex interactions between solutes and solvents and their impact on absorption phenomena. Solute–solvent repulsion effects help explain several factors that impact absorption spectra. This is important for scientists, who can use the knowledge this study imparts to develop more accurate analyses in various scientific applications.
(1) Roszak, K.; Maciejewski, A.; Katrusiak, A.; Krystkowiak, E. Solute − solvent repulsion effects on the absorption spectra of anthracene in n-hexane investigated under high pressure. Spectrochimica Acta Part A: Mol. Biomol. Spectrosc. 2023, 299, 122822. DOI: 10.1016/j.saa.2023.122822